A fuel cell, in one embodiment, consists of a support tube over which is deposited an air electrode layer, followed by a layer of an electrolyte, and then a layer of a fuel electrode. The fuel electrode material generally consists of a sintered powdered metal or metal oxide. Since the fuel cell operates at high temperatures, the materials selected for use in it must be compatible in chemical, electrical, and physical-mechanical characteristics, such as thermal expansion. A thermal expansion mismatch between two contacting cell components is a serious concern as it may lead to cracking or delamination.
While the best material for fuel electrodes in a fuel cell is currently believed to be nickel or cobalt, there is a discrepancy factor of about 1.6 between the coefficient of thermal expansion of these metals and the coefficient of thermal expansion of materials used to make the electrolyte in the fuel cell. As a result, when the fuel cell is thermally cycled between room temperature and operating temperature, the interface between the electrolyte and the fuel electrode is severely stressed, which can lead to a separation and loss of contact between these two components. Since close contact is essential to obtain a low cell resistance and therefore a high performance, this problem prevents the fuel cell from operating efficiently.
Besides the purely thermal-mechanical problems that exist at the fuel electrode-electrolyte interface, there are electrochemical effects which also reduce the mechanical stability at the interface. At the interface, fuel, such as hydrogen and carbon monoxide, react with oxygen ions from the electrolyte to form water vapor and carbon dioxide. If the water vapor cannot easily escape from the area where it is formed, it can force the electrode off the electrolyte. Other forces are also at work at the interface whih are not yet well understood and which affect the adherence of the electrode to the electrolyte, such as the wetting behavior of metals, which is a function of the gas composition and its oxygen activity. Until now, the problem of the adherence of the electrode to the electrolyte has been attacked by sintering, plasma (flame) spraying, and sputtering the electrode onto the electrolyte. These methods have met with little success, and often they are not feasible because they are uneconomic and do not produce electrodes with the correct pore structure. The sintering of slurry coatings of zirconia, mixed with nickel or cobalt oxides, has been attempted with little success because, in the preferred fuel cell structure, the fuel electrode application is the last step in the sequence of several cell component fabrication steps. In order to prevent damage to the other components, the fuel electrode sintering temperature in this case must be restricted to a maximum of 1300.degree. to 1350.degree. C., which is too low to form a good electrode bond to the electrolyte using that composition.